Process for bleaching and sizing paper pulp



PROCESS FOR BLEACi-HNG AND SIZING PAPER PULP Charles R. Outterson, Richmond, Va., and Richard S. Bell and Henry W. Ellerson, Jr., Roanoke Rapids, N. C., assignors to The Albermarle Paper ManufacturingCompany, Richmond, Va., a corporation of Virginia No Drawing. Application March 2, 1956 Serial No. 568,962 I 12 Claims. (Cl. 9221) This invention relates to a new and improved process for bleaching sulfate paper pulp.

It has been the practice hitherto to bleach the paper pulp and then to wash it thoroughly to remove chlorinated lignins, oxidized color bodies, soluble salts, such as calcium salts produced by calcium hypochlorite bleaching, and other impurities prior, to sizing of the paper furnish. Such separate preliminary bleach and wash operations have been the rule whether the bleaching is multi-stage to produce substantially fully bleached paper or single-stage to produce semi-bleached paper and require the installation and maintainance of expensive chlorination and wash equipment, additional manpower and considerably increased processing time. Where 'a' semibleached pulp is suitable for the particular use, the added costs of separate bleaching and washing treatments is often prohibitive.

The object of this invention is to provide a process whereby sizing of sulfate paper pulp can be carried out either during the bleaching procedure or subsequent to bleaching but in the presence of the bleaching reaction by-products in such manner that separate bleaching equip ment is rendered unnecessary.

Another object is to provide a process whereby bleaching and sizing of sulfate paper pulp can be performed in a single unitary operation.

Still another object is to eliminate washing of the bleached pulp as a separate operational procedure.

Other objects include the manufacture of semi-bleached kraft paper having excellent size and strength properties and good color at greatly reduced cost.

Broadly speaking, our process comprises admixing the sulfate pulp with hypochlorite bleach, such as calcium, sodium or potassium hypochlorite, allowing sufiicient time for the bleaching oxidation and chlorination action of the hypochlorite to become effective, and thenadding alum both to release the chlorine remaining in the residual hypochlorite and to set the rosin size. Rosin size is added at any time prior to addition of the alum or can be introduced subsequent to alum addition. Washing of the bleached pulp as a separate operation, whether the rosin is added in the presence of the hypochlorite or subsequent to alum addition, can be dispensed with since we have found that the aqueous dilution of the pulp which is required for sheet formation on the paper making ma,

chine can serve as an adequate wash mechanism if certainprecautions are observed as will subsequently be described. Although, as aforementioned, other hypochlorites can be employed with equally satisfactory results, We prefer, in general, to employ calcium hypochlorite because of its low cost and ease of handling.

The amount of hypochlorite added to the sulfate pulp is determined largely by the bleach demand of the pulp, the degree of brightness desired and the amount of time which can be allowed for action of the bleach in terms of available equipment and economical processing. In general, we prefer to employ the hypochlorite inan amount which provides suflicient available. chlorine to satisfy up nited States Patent 2,878,119 Patented Mar. 17, 1959 ice to about 70% of the bleach demand of the pulp since the pulp reacts with the bleach in such proportions quite, rapidly, generally within a period of about /2 to; hours. and these amounts generally produce a semi-bleached paper of good brightness for most purposes. Itwill be understood, however, that larger proportions ofthe hyjpochlorite as, for example, up to the total bleach demand, may be'used but, because of the slowness of reaction after, bleachability of the pulp has beenreduced byabout to 70%, either considerably longer periods of time must be allowed for more complete bleaching action or sub: stantial waste of unreacted bleach will be incurred.

Bleach requirement or bleachability of the pulp can ,be, determined by any of the well known standard i'nethods.v as, for example, by determination of the permanganate number or the number of milliliters of N/lO .permann ganate consumed by 1 gram of oven-dry pulp. The per: manganate' number can be converted into approximately equivalent chlorine in terms of hypochlorite for bleaching by use of the proper factor for the particular type of pulp.

The hypochlorite can be added to the, pulp, if desired;

the beater for the addition of the rosin size and .alurn.

However, from the point of view of economic equipment utilization, we prefer toconduct the bleaching opera tion in the beater.

The hypochlorite can be introduced into the beaterconcurrently with the sulfate paper pulpor after the beater has been filled; Reaction of the hypochlorite with the lignins and other color-forming bodies in the pulp propceeds quite rapidly until an amount of hypochlorite providing available chlorine equivalent to about 60 to 70%; of the bleach demand is consumed in both oxidation and chlorination reactions. If more hypochlorite is present, reaction will generally continue much' more slowly beyond this point. i

The normal alkalinity of the sulfate pulp (generally in the neighborhood of about pH 9) provides substantially optimum conditions for the action of the hypochlorite, both from the point ,of view of, speed of reaction and minimizing degradation of the pulp. As reaction with; the hypochlorite continues pH decreases, generally to products.

The rosin size can be introduced; into the paper,

furnish in the beater at' any point prior to addition ofthe alum. Itcan be added before thehypochlorite, concur: rently with the hypochlorite or subsequent to the addi tion of the hypochlorite. The rosin is entirely compati ble with the hypochlorite in the pulp mixture. In som cases as, for example, where the paper pulp is passed in a continuous process through Jordan refiners, the rosin size can be added to the pulp in the refiners subsequent, to the addition of the alum in the beater. In suchcase the standard paper making practices employed where rosin is added after the alum should be followed. The rosin size should desirablybe introduced indilutedforrn as, for example, about l% solidsv concentration, to prevent loss of sizing uniformity because of rapid precipitationl small to substantial amounts of free rosin and free rosin- It will be under-'1 size emulsions, such as Bewoid size. stood that the term rosin as employed in the specification and claims encompasses the various types of rosin size,'as aforedescribed. The rosin size furthermore can be em ployed in conjunctionwith such other sizing components as 'wax-emulsionsand the like "depending on the specific properties required in the paper.

It has generally been the consensus of opinion hitherto that the presence of calcium saltsiadversely affects the rosin sizingoperation. It would appear, therefore, that the calcium salts, such as calcium sulfate formed upon addition of alum, which are a by-product of calcium hypochlorite bleaching, would exert a deleterious action on the rosin sizing.- We have found, however, that sizing i s-actually improved when it is carried out in conjunction with the hypochlorite bleaching. In other words, with a"'given pulp, type and amount of size, improved sizing results, as for example, in terms of water resistance, are

obtained when the hypochlorite bleach is applied during the sizing operation. This is an unexpected phenomenon, which, while we cannot at present satisfactorily explain it, isfan important advantage of our process.

"f'lhe amount of rosin size employed is that normally used in the paper making art for the particular type of pulp and desired finished paper characteristics.

"-Other'm 'aterials which are ordinarily incorporated into the paper stock in the beater can be introduced as well as the rosin size, such as dyestufifs and fillers, so long as they'are not chemically afiected by the hypochlorite. The dyestuffs employed, for example, should be of the acid-fast type.-

The action'of the hypochlorite on the pulp is allowed to continue in the heater for a suitable length of time. The particular reaction period is determined by a number of factors such as the amount of hypochlorite employed, the temperature and concentration of the pulp, degree of brightness desired in the finished paper, practical considerations of permissible processing time and the like.

heamm which is then added to the beater serves a purpose. It eliminates the residual hypochlorite rel'mainingin the pulp by reactingwi'th it to release free chlorine which acts on the pulp to produce additional bleaching action, and it provides for precipitation or setting the rosin size whether the latter is introduced before'or after. The amount of alum required to remove the residual hypochlorite over and above that normally employed" to precipitate the size is generally quite small. We have found that the addition of alum in an amount sufficient todecrease pH of the pulp mix to about 6.0 or 6; is generally adequate both for satisfactory rosin fixing andhypochlorite elimination, though more may he added.

Concentration of the pulp can be at any workable level which can satisfactorily be handled by the available equipment. In most cases, the heater is geared to handle pulp-concentratidns of about 3 to 5 percent. Bleaching of the pulp according to our process is very satisfactory at these concentrations, particularly with regard to uniformity.

- The treated pulp, after leaving the beater, can be subjected to ordinary paper-making routine. It can, for example, be dumped into a stud? or heater chest for routine storage and then pumped through refiners, where as aforesescribed, rosin size can be introduced if sizing has not been performed in the beater during the hypochlorite treatment, thence to another stuff or machine chest and on to the paper making machine. If the stock is not subjected to refiner treatment, it can be passed directly from the heater or beater chest to the paper making machnie. I

One advantage of the standard practice of temporarily storing the beater contents in a beater chest prior to sheet formation on the paper making machine stems from the fact that this storage time gives such chlorine as has been released from the residual hypochlorite by the alum opportunity to act on and be absorbed by the pulp for further bleaching action.

uWhen the bleached and sized pulp reaches the papermaking machine it is subjected to routine dilution in the head box for sheet formation on the wire screen. In routinepaper making practice the pulp is diluted to a ,4 consistency of about 0.2 to 0.5 although concentration may be as low as 0.1% and as high as 1% depending on the particular type of paper being made and the characteristics of the particular pulp such as type and length of the fibers and the like.

Wehave found that the water which is added to dilute the paper making stock prior to sheet formation and then drained off the paper machine wire provides adequate Washing of the bleached pulp and removes a sufficient amount of the bleaching by-products and impurities to prevent any substantial color reversion. This method possesses the important advantage of eleminating a separate washing procedure along with the expensive equipment and additional processing time which this would require. 7

Fresh water can be employed for dilution of the paper making stock, if desired, for somewhat more thorough washing. However, the process can be more economically practiced by recirculating the white water efiluent for stock dilution so long as care is exercised to avoid the use of water which has become substantially saturated with the bleach impurities since this will reduce the washing effect to the point where substantial color reversion will occur. This can be avoided by addition of some fresh water to the recirculated white water efliluent as required.

The semi-bleached papers formed by our process are of excellent quality with regard to strength, size properties and brightness. The ordinary brown color of unbleached kraft papers is reduced generally to a bright canary yellow. They are highly satisfactory for use in specialty wrapping papers, such as butcher paper, manila envelopes and folders, fiber cartons 'and 'the like.

Example I 800 lbs. of air dry sulfate pulp in the form of slush was furnished to a beater and reduced to a consistency of 4 While the pulp was being introduced into the beater, a bleaching solution of calcium hypochlorite containing approximately 30 gms./liter of chlorine was also run in. The total quantity of bleach was equivalent to 4% chlorine based on the air dry fiber. Maleic fortified rosin size was then added to the beater contents in an amount corresponding to 1% by weight of the air dry fiberj 0.5% emulsified wax size by weight of the air dry fiber was then introduced. Beater action was continued for about 1 hour, after which alum was added in amountsufiicient to precipitate the rosin and wax sizes and to acidity the entire beater contents to approximately pH 6.0. i

The bleached and sized fiber was passed into a heater chest and then on to a Fourdrinier paper machine where it was diluted to a consistency of about 0.5% prior to sheet formation. Water drained from the sheet on the paper wire was recirculated to the head box for stock dilution.

Paper was made having a basis weight of 40 lbs. per ream of 500 sheets, size 24 in. by 36 in. Tests were made in accordance with ASTM designation D-779-54 to determine the degree of sizing. Results of 38 to 40 seconds were obtained, indicating a high degree of water resistance. .The color of the paper was changed from brown to a pleasing canary yellow and brightness was improved by about 10 points, namely from Hunter 34 to 44. The permanganate number of the pulp was reduced from 26 to 17.

Example II The same procedure was followed as in Example I except that the amount of calcium hypochlorite added was increased to the equivalent of 8% chlorine based on air dry fiber and the pulp was retained in the heater for minutes.

The degree of sizing of the paper was about the same asiziExample-I." Brightness'was improved by about 17 points, namely from Hunter 34 to 51 and permanganate number of the pulp was reduced from 26 to 12.

Although this invention has been described with reference to illustrative embodiments thereof, it will be apparent to those skilled in the art that it may be embodied in other forms but within the scope of the appended claims.

We claim:

1. A process for bleaching and sizing sulfate paper pulp which comprises admixing an aqueous slurry of the pulp with hypochlorite bleach, permitting the hypochlorite to react with the pulp for at least /2 hour and then adding alum in an amount sufiicient to react with residual hypochlorite and to precipitate the rosin size, introducing rosin size into the pulp at any time prior to dilution of the pulp for sheet formation, whereby sizing of the paper pulp is accomplished in the presence of said hypochlorite bleach or its reaction products, then diluting said pulp with water to the consistency requisite for paper sheet formation, the maximum concentration of the pulp being about 1%, forming paper sheets with said diluted pulp and removing the water after said sheet formation, said dilution water serving as the only wash for said treated pulp.

2. The process of claim 1 in which the pulp concentration when treated with the hypochlorite is about 3 to 5%.

3. The process of claim 1 in which the hypochlorite is calcium hypochlorite.

4. The process of claim 2 in which the pulp concentration when treated with the hpochlorite is about 3 to 5%.

5. The process of claim 2 in which the calcium hypochlorite is added in an amount comprising up to the equivalent of about 70% of the bleach demand of the pulp.

6. The process of claim 5 in which the pulp concen- 7. A process for bleaching and sizing sulfate paper pulp which comprises admixing an aqueous slurry of the pulp with hypochlorite bleach and rosin size, permitting the hypochlorite to react with the pulp for at least 16 hour and then adding alum in an amount sufficient to react with residual hypochlorite and to precipitate the rosin size, the rosin size being introduced into the pulp at any time prior to addition of the alum, whereby sizing of the paper pulp is accomplished in the presence of said hypochlorite bleach or its reaction products, then diluting said pulp with water to the consistency requisite for paper sheet formation, the maximum concentration of the pulp being about 1%, forming paper sheets with said diluted pulp and removing the water after said sheet formation, said dilution water serving as the only wash for said treated pulp.

8. The process of claim 7 in which the pulp concentration when treated with the hypochlorite is about 3 to 5%.

9. The process of claim 7 in which the hypochlorite is calcium hypochlorite.

10. The process of claim 9 in which the pulp concentration when treated with the hypochlorite is about 3 to 5%.

11. The process of claim 9 in which the calcium hypo chlorite is added in an amount comprising up to the equivalent of about of the bleach demand of the pulp.

12. The process of claim 11 in which the pulp concentration when treated with the hypochlorite is about 3 to 5%.

Casey: Pulp and Paper, vol. I, Interscience Publishers, 1952, page 273, 1st paragraph.

Hamburg Apr. 27, 1954 

1. A PROCESS FOR BLEACHING AND SIZING SULFATE PAPER PULP WHICH COMPRISES ADMIXING AN AQUEOUS SLURRY OF THE PULP WITH HYPOCHLORITE BLEACH, PERMITTING THE HYPOCHLORITE TO REACT WITH THE PULP FOR AT LEAST 1/2 HOUR AND THEN ADDING ALUM IN AN AMOUNT SUFFICIENT TO REACT WITH RESIDUAL HYPOCHLORITE AND TO PRECIPITATE THE ROSIN SIZE, INTRODUCING ROSIN SIZE INTO THE PULP AT ANY TIME PRIOR TO DILUTION OF THE PULP FOR SHEET FORMATION, WHEREBY SIZING OF THE PAPER PULP IS ACCOMPLISHED IN THE PRESENCE OF SAID HYPOCHLORITE BLEACH OR ITS REACTION PRODUCTS, THEN DILUTING SAID PULP WITH WATER TO THE CONSISTENCY REQUISITE FOR PAPER SHEET FORMATION, THE MAXIMUM CONCENTRATION OF THE PULP BEING ABOUT 1%, FORMING PAPER SHEETS WITH SAID DILUTED PULP AND REMOVING THE WATER AFTER SAID SHEET FORMATION, SAID DILUTION WATER SERVING AS THE ONLY WASH FOR SAID TREATMENT PULP. 